1. Field of the Invention
This invention relates to an electroluminescent (EL) display device, specifically to an EL display device that includes an ultraviolet protection film.
2. Description of the Related Art
FIG. 6A shows a plan view of a conventional passive-type EL display device, FIG. 6B shows a sectional view of the EL display device along line C—C of FIG. 6A, FIG. 6C shows a sectional view along line D—D of FIG. 6A, and FIG. 6D shows a partially expanded view of FIG. 6C with a conventional ultraviolet cut-off structure.
As shown in FIG. 6A, linear anodes 20, which extend vertically in the figure and are formed from ITO (indium tin oxide) that is a transparent conductive material, and linear cathodes 30, which extend horizontally in the figure, are placed on an insulating substrate 10. At the intersections of the vertical anodes 20 and the horizontal cathodes 30, organic EL layers 25 having light emitting layers formed from an organic material are provided between the anodes 20 and the cathodes 30. This three-layer structure forms an organic EL element 31.
As shown in FIG. 6D, the anode 20 formed of a transparent conductive film is provided on the insulating substrate 10. The organic EL layer 25 formed on the anode 20 includes a hole transporting layer 21 having a first hole transporting layer made of MTDATA (4,4′,4″-tris(3-methylphenylphenylamino)triphenylamine) and a second hole transporting layer made of TPD (N,N′-diphenyl-N,N′-di(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine), a light emitting layer 22 made of Bebq2 (10-benzo[h]quinolinol-beryllium complex) containing quinacridone derivative, and an electron transporting layer 23 made of Bebq2. Furthermore, the cathode 30 made of aluminum (Al) is formed on the EL layer 25. This cathode 30 is an opaque conductive film, and light emitted from the light emitting layer 22 passes through the insulating substrate 10 as the light for the display device. An insulating film 24 is placed between the anode 20 and the cathode 30.
A sealing substrate (not shown) is attached to the insulating substrate 10 to cover the cathode of the insulating substrate 10, and the organic EL element 31 as well as associated thin film transistors (TFT) are sealed between this sealing substrate and the insulating substrate 10. This sealing substrate may be a metal or a glass. The organic EL layer 25 is not resistant to moisture, and therefore, it is desirable that the sealing is completed as quickly as possible.
External light 100 also enters the display device, and this light is reflected by the cathode 30 formed of an Al layer. This reflected light adds to the brightness of the display device. For example, in a black representation by the display device, a proper black representation cannot be obtained. Therefore, as shown in FIG. 6D, a retardation film 51 and a circular polarizer 52 are provided on the insulating substrate to cut the light reflected by the cathode 30 to achieve the proper black representation.
Since the transmission of the circular polarizer 52 is approximately 40% through 50%, only about a half of the light generated at the light emitting layer 22 is used as display light. Accordingly, this low light transmission of the circular polarizer 52 coupled with the retardation film 51 reduces the amount of ultraviolet rays that reaches the organic EL layer 25. The conventional display device relies on the circular polarizer 52 and the retardation film 51 to maintain a proper life time of the organic EL layer, which is subject to an accelerated light emission character degradation upon illumination by ultraviolet rays.